The presence of toxic materials in surface and groundwater has been recognized as a problem of growing significance. Toxic chemicals are being detected in drinking water supplies, rendering them unusable. Most of the chemicals are organic chemicals that are relatively stable in the environment. Toxic removal from water poses economic and logistical challenges since the contaminants are usually present in low concentrations in a large volume of water. A number of technologies, including air stripping, carbon adsorption, and chemical oxidation, have been used for removal of toxic organics. Air stripping and activated carbon adsorption techniques have the disadvantage of concentrating the contaminants in a separate, usually solid, phase, which still requires further disposal.
In contrast, chemical oxidation is a destruction, as opposed to a transfer, process. In chemical oxidation, toxics are oxidized to non-toxic or less-toxic products. If complete reaction is achieved, termed complete mineralization, the final products of oxidation are carbon dioxide, water, and salts. UV-oxidation is an advanced oxidation process in which a strong oxidizing chemical, typically ozone or hydrogen peroxide, is added to the contaminated water and irradiated with ultraviolet (UV) light. For reasons of cost and practicality, hydrogen peroxide, conventionally termed peroxide, is generally the preferred oxidizer.
When hydrogen peroxide (H.sub.2 O.sub.2) absorbs UV radiation, it breaks up (is photolyzed) into highly-reactive hydroxyl radicals (OH.) that react with and oxidize many organic chemical compounds. In conventional UV-oxidation systems, the source of UV radiation is typically an elongated tubular mercury vapor arc lamp excited by alternating current conducted between electrodes by a carrier gas, typically argon. In order to make use of all the radiation emitted by the lamps, typically the lamps are enclosed for protection in immersion wells made of quartz, a material that transmits UV light with minimal absorption, and immersed in a reactor vessel containing the water to be treated. An example of a typical system using immersed mercury arc lamps is described in U.S. Pat. No. 5,151,174 to Wiesmann.
UV radiation is also used without an oxidizer for disinfection of water as an alternative to chlorination. Immersion systems are typical, but other systems having a mercury arc lamp mounted external to the water have also been developed. In these systems, water flows through a chamber in a direction that is at a right angle to the UV radiation, or water flows through a transparent tube mounted in close proximity to the lamp.
UV-oxidation and UV-disinfection processes can be configured in batch or continuous-flow operations. For large-scale treatment facilities, continuous-flow configurations are desirable to achieve the high throughput required. However, in flow-through reactors, incoming solution is constantly mixing with treated solution, so there is always a chance that partially-treated solution can be output. Therefore, baffles or serpentine-shaped reaction chambers, for example, as in U.S. Pat. No. 3,634,025 to Landry, are often included in the reactors to channel the solution and increase residence time to ensure maximum exposure to UV radiation. Another example, U.S. Pat. No. 5,785, 845, to Colaiano, also describes such a reactor. In some implementations, ozone is used instead of hydrogen peroxide and is bubbled through the water to dissolve ozone in the water. The ozone molecule reacts with water molecules to form peroxide for photolysis by the UV radiation. Treatment methods using ozone are described, for example in U.S. Pat. No. 4,230,571 to Dadd and in U.S. Pat. No. 5,494,576 to Hoope et al. Other implementations have used chemical catalysts and require acidification of the water, as is the case in U.S. Pat. No. 3,819,516 to Murchison, et al.
While UV-oxidation is potentially a very attractive technology for treating contaminated groundwater because of the possibility of complete destruction of toxic chemicals, a number of drawbacks of the current systems have limited its widespread use. First, the energy used to power the UV radiation source may not be used efficiently in current systems because very little of the spectral output of conventional mercury arc lamps is in the short-wavelength range that is best absorbed by peroxide. This situation becomes even more pronounced at very low concentrations of peroxide, which would be used for treating waters with very low concentrations, less than 1 ppm, of organics. At the low levels of peroxide that would be used only the very short UV wavelengths are absorbed and utilized. Furthermore, multiple lamps in arrays are often needed to provide enough useful energy to achieve high throughput rates, and the lamps steadily degrade in output power and efficiency in continuous service. This combination has resulted in systems that are inefficient in terms of the amount of toxics removed for the amount of energy input, resulting in higher overall cost when compared to transfer technologies such as carbon adsorption and air stripping. Second, the quartz immersion wells are costly, require replacement when used with high-power lamps, and easily become fouled with substances from the liquid being treated, which necessitates frequent cleaning that adds to the operating cost of the system. Between cleanings any fouling present reduces the transmission of UV radiation and further diminishes efficiency. A third drawback is that reactor designs tend to be complicated to accommodate arrays of lamps and to provide baffles or complicated flow patterns, which, as discussed above, are often needed to maximize residence time of the solution and exposure to UV radiation. Complexity increases capital cost and maintenance. The use of other reactants such as ozone, or catalysts such as Fenton's reagent is not practical for high-volume operations due to cost.
In typical high-volume UV water disinfection applications, very large arrays, often containing thousands of conventional mercury arc lamps, are used. These arrays can weigh several tons. The water flows through the array, around immersed lamps. As the lamps age, their UV output diminishes steadily, resulting in substantial gradations of effectiveness throughout the array. For this reason it is not possible to ensure that all of the water is exposed to the same amount of UV energy and is treated sufficiently. Thus systems typically have a tolerance level for output quality, allowing a specified level of live bacteria to still be present in treated water. Such systems are expensive to build, difficult and expensive to maintain, and consume large amounts of electrical energy. For example, 700 kilowatts for a system that is capable of disinfecting 1 million gallons per day is typical. Because the lamp arrays are very large and massive, it is not practical to shut down the system to remove the array from the reactor and change individual lamps. In some systems all lamps are replaced on a regular schedule at the same time, helping to ensure that there will be as few individual variations in output as reasonably possible. Total replacement is, however, an expensive solution.
In other water disinfection systems, a mercury arc lamp is mounted to the side of a quartz tube through which water flows, as in U.S. Pat. No. 4,274,970, to Beitzel. In these systems, it is very difficult to ensure that all of the water is exposed to UV equally or sufficiently due to the uneven distribution of UV radiation. Therefore, a slow flow rate, resulting in a long residence time, is necessary in these systems.
What is needed is a way to utilize ultraviolet radiation for UV-oxidation or UV-disinfection of water that assures even exposure of the water to the radiation. Furthermore, what is needed is a UV light source that uses energy more efficiently than the sources commonly employed and a reactor design that avoids complications such as quartz immersion wells and baffles or serpentine chambers. It would be desirable to provide a UV-oxidation or UV-disinfection system that is more efficient to build and operate than the current technologies. Further, it would be desirable for the system to be scaleable so that the benefits of UV-oxidation and UV-disinfection can be realized in practice for high-volume water treatment applications.